Vitamin D (1,25(OH)2D3, calcitriol) and its analogues show substantial promise for the treatment and prevention of prostate cancer. These antineoplastic effects of calcitriol are mediated through either apoptosis or decreased proliferation in a number or experimental systems; however, the mechanisms that regulate these effects remain unknown. Recently, in a placebo-controlled clinical trial, we discovered that calcitriol downregulates a novel homeobox gene in human prostate cancer. This gene belongs to a family of homeobox genes that act as transcriptional activators and repressers and play multiple roles in embryonic development. Subsequent in vitro studies showed that calcitriol also reduced the expression of this gene in androgen-sensitive LNCaP prostate cancer cells. Downregulation of the gene by RNA interference induced apoptosis of LNCaP cells in a time-dependent manner. Therefore we hypothesize that this gene regulates calcitriol-induced apoptosis of human prostate cancer cells and is a potential new therapeutic target. To determine the role this gene plays in regulating apoptosis and mediating calcitriol-induced apoptosis in prostate cancer we will examine the effect of its downregulation by siRNA in LNCaP on pro- and anti- apoptotic proteins that are affected by calcitriol and compare the effects of calcitriol to those of direct gene downregulation. The role of the p53 tumor suppressor in the induction of apoptosis in our experimental system will be tested using LNCaP cells stably expressing p53 dominant-negative mutants. We will also determine if enforced overexpression of this gene confers resistance to calcitriol-induced apoptosis. In an orthotopic mouse model, we will determine the effect of this gene on tumorigenicity, prostate tumor growth, apoptosis, and angiogenesis. This work will generate new knowledge about mechanisms of calcitriol- induced apoptosis and will develop a novel target for cancer treatment. New treatment targets are needed to develop new drugs to treat prostate cancer, the most commonly diagnosed cancer and the second-leading cancer killer in US men. [unreadable] [unreadable]